"Sheeting" is known to occur in the reaction chamber of gas phase polymerization reactors. Sheeting is the adherence of fused catalyst and reactant particles to the walls of the reaction chamber. Sheeting generally is undesirable because it can disrupt temperature control of the reactor, can result in the creation of "chunks" of polymer material that fall off of the walls and clog a distribution plate (which maintains a gas stream for a fluidized bed in the reactor), and can result in the creation of "off-specification" polymer material that mixes with "on-specification" polymer material to provide an unacceptable batch of polymer material.
It is known to reduce sheeting during polymerization (i.e., polymer preparation) in a fluidized bed by introducing an additional substance to the materials used to create the polymer. The materials used to create the polymer typically are the reactant (e.g., a monomer) and the catalyst. The additional substance is introduced only to reduce sheeting. Additional substances that have been used to reduce sheeting are: charge-generating chemical additives such as magnesium oxide and vandium oxide; inert materials in particle or powder form such as silica; water; methanol; chromium-containing compounds; and "anti-stats" which are antistatic materials having a small amount of conductive material to aid in charge dissipation. Some of these substances, such as water and methanol, are known reaction poisons. It also is known to reduce sheeting during polymer preparation in a fluidized bed by introducing a tangential flow into the reaction chamber to remove build-up on the walls of the chamber.
Before sheeting is reduced in any of the ways mentioned in the preceding paragraph, it generally first must be detected. It is known to detect sheeting in a reaction chamber of a polymerization reactor by measuring the "static electricity" voltage in the reaction chamber with a fragile probe and a voltage divider circuit. The fragile probe generally is expensive to manufacture. It is believed, and generally described in the prior art, that high static voltages (e.g., 10 kilovolts or more) in the reaction chamber are indications of sheeting and/or other problems occurring or about to occur in the chamber. One end of the probe, which typically is a metal rod surrounded by a fragile insulating ceramic, protrudes into the reaction chamber and the other end of the probe connects to the circuit. The function of the circuit is to divide down most of the large voltage and thus allow a small voltage to be measured. The higher the voltage measured with the circuit, the higher the static voltage in the reaction chamber and the more sheeting in the reaction chamber that may result.
It also is known to use the fragile probe and voltage divider circuit combination described above to measure the static electricity voltage in fluidized beds generally. High voltages in a fluidized bed are believed to cause, or contribute to, problems with the functioning of the bed and problems with the material being mixed or created with the bed. Fluidized beds are used in a variety of industries including pharmaceutical, food, and metal to mix and react substances and make various drugs, foods, and other products.
Because it is believed that high static voltages are possible in the fluidized beds, the insulator used for the probe is a high quality insulating material, such as ceramic which is expensive and fragile and thus prone to breakage, to prevent arcing from the probe to ground. Also, because of the high static voltages, special safety equipment must be used such as a grounding bar outside the reactor to prevent accidental shocks to anyone making a connection to the reactor and to prevent sparks or arcing which might result in an explosion of various substances in the vicinity and atmosphere near these reactors. It is typical to have ignitable or explosive substances (e.g., gases, chemicals, etc.) near the reactor.
A way is needed to detect problems in fluidized beds (e.g., sheeting in a reaction chamber of a polymerization reactor) without using expensive-to-manufacture, fragile insulating materials such as ceramics on the probe and without the need for special safety equipment such as grounding bars and spark protectors.